1. Field of the Invention
This present invention relates to the field of programmable robotic manipulators and assist devices, and more particularly to robotic manipulators and assist devices that can interact with human operators for the manipulation of heavy payloads.
2. Description of Related Art
In an industrial application such as a manufacturing assembly line or a general material handling situation, the payload may be too large for a human operator to move without risking injury. Even with lighter loads, it may be desirable to provide mechanical assistance to a human operator in order to allow more rapid movement and assembly and to avoid strain and fatigue. Thus, a great deal of industrial assembly and material handling work is done with the help of assist devices, such as overhead bridge rail systems.
Overhead bridge rail systems are also known in the art as “bridge cranes” or “xy rail systems.” One type of powered overhead bridge crane runs on I-beams and are typically used for heavy loads. Powered bridge cranes are relatively slow and are usually directionally controlled by a human-controlled pushbutton-type device that is coupled to the crane. Manipulating the system to get the payload to its desired position can be a challenge due to the slow speed of the crane and the tedious manipulation of the input device required to yield the desired path.
Also, there are unpowered overhead rail systems that are typically used for lighter loads. Unpowered overhead rail systems utilize low-friction rails and are moved by the direct application of the user's force to the payload. Unpowered rail systems are typically faster and easier to use, and allow greater operator dexterity.
However, a number of problems plague unpowered overhead rail systems. First, it can be difficult to accelerate the payload. Frequently, this involves forward pushing, which uses the large muscles of the lower body. Even so, considerable effort is required to accelerate larger payloads that are typically above about 200 lbs. Second, controlling or steering the motion of the moving payload is an even greater problem, as it requires pulling sideways with respect to the payload's direction of motion, generally using the smaller muscles of the upper body and back. Third, stopping the motion of the payload, as well as the crane itself, is also a significant problem. Even if the operator pulls hard enough to stop the payload motion, the crane will continue traveling, thereby requiring an extra pulse of stopping force.
Anisotropy is a further problem with an unpowered system. Although a low-friction design is used, both the friction and the inertia are greater in the direction in which the payload has to carry the whole bridge rail with it than in the direction in which the payload simply moves along the bridge rail. Anisotropy produces an unintuitive response of the payload to applied user forces, and often results in the user experiencing a continuous sideways “tugging” as the payload moves, in order to keep it on the desired path.
In conventional rail systems, if the operator suddenly stops moving the payload, for unpowered rail systems, or stops commanding the motion of the overhead carriage, for powered bridge cranes, the payload may tend to swing up and back below its support point. Swinging causes delay and difficulty in positioning the payload.